U.S. patent application number 13/082946 was filed with the patent office on 2012-10-11 for variable durometer lumbar-sacral implant.
This patent application is currently assigned to KYPHON SARL. Invention is credited to Eric C. Lange.
Application Number | 20120259364 13/082946 |
Document ID | / |
Family ID | 46966685 |
Filed Date | 2012-10-11 |
United States Patent
Application |
20120259364 |
Kind Code |
A1 |
Lange; Eric C. |
October 11, 2012 |
VARIABLE DUROMETER LUMBAR-SACRAL IMPLANT
Abstract
Medical devices for the treatment of spinal conditions are
described herein. The medical device includes a main body that is
adapted to be placed between the L5 vertebra and the sacrum so that
the main body acts as a spacer with respect to the L5 vertebra and
the sacrum to maintain distraction therebetween when the spine
moves in extension. The main body is formed from a material having
a gradual variation in modulus.
Inventors: |
Lange; Eric C.; (Pleasanton,
CA) |
Assignee: |
KYPHON SARL
Neuchatel
CH
|
Family ID: |
46966685 |
Appl. No.: |
13/082946 |
Filed: |
April 8, 2011 |
Current U.S.
Class: |
606/246 |
Current CPC
Class: |
A61B 17/7062 20130101;
A61B 17/7067 20130101; A61B 17/7055 20130101 |
Class at
Publication: |
606/246 |
International
Class: |
A61B 17/70 20060101
A61B017/70 |
Claims
1. A device, comprising: a front face; a rear face; an upper body
portion defining an upper saddle wherein the upper body portion has
a first modulus; a lower body portion defining a lower saddle; the
upper body portion separated from the lower body portion by an
axial plane wherein the lower body portion has a second modulus
wherein the second modulus is greater than the first modulus; the
lower body portion including a left lower lobe and a right lower
lobe, each lobe being adjacent to an opposite side of the lower
saddle; a left sidewall, a right sidewall and a sagittal plane
dividing the device into a left part and a right part, the left
sidewall and the right sidewall each extending from the upper body
portion to the lower body portion and extending away from the
sagittal plane in a direction from the upper body portion to the
lower body portion such that a first distance between the left
sidewall and the right sidewall adjacent to the upper body portion
is less than a second distance between the left sidewall and the
right sidewall adjacent to the lower body portion; and a left
channel extending through the device in the left lower lobe and a
right channel extending through the device in the right lower
lobe.
2. The device of claim 1 wherein the device has a gradual variation
in modulus such that the modulus gradually increases from the upper
body portion to the lower body portion.
3. The device of claim 1 wherein the device has a gradual variation
in modulus such that the modulus gradually increases from a central
portion of the device to the left sidewall and the right
sidewall.
4. The device of claim 2 wherein the device has a gradual variation
in modulus such that the modulus gradually increases from a central
portion of the device to the left sidewall and the right
sidewall.
5. A device, comprising: an upper body portion defining an upper
saddle wherein the upper body portion has a first modulus; and a
lower body portion defining a lower saddle wherein the lower body
portion has a second modulus wherein the second modulus is greater
than the first modulus.
6. The device of claim 5 wherein the device has a gradual variation
in modulus such that the modulus gradually increases from the upper
body portion to the lower body portion.
7. The device of claim 5 wherein the device has a gradual variation
in modulus such that the modulus gradually increases from a central
portion of the device to a left sidewall and a right sidewall.
8. The device of claim 6 wherein the device has a gradual variation
in modulus such that the modulus gradually increases from a central
portion of the device to a left sidewall and a right sidewall.
9. A device, comprising: a front face; a rear face; an upper body
portion defining an upper saddle; a lower body portion defining a
lower saddle; the upper body portion separated from the lower body
portion by an axial plane; the lower body portion including a left
lower lobe and a right lower lobe, each lobe being adjacent to an
opposite side of the lower saddle; a left sidewall, a right
sidewall and a sagittal plane dividing the device into a left part
and a right part, the left sidewall and the right sidewall each
extending from the upper body portion to the lower body portion and
extending away from the sagittal plane in a direction from the
upper body portion to the lower body portion such that a first
distance between the left sidewall and the right sidewall adjacent
to the upper body portion is less than a second distance between
the left sidewall and the right sidewall adjacent to the lower body
portion; a central body portion disposed between the upper body
portion and the lower body portion and between the left part and
the right part; and a left channel extending through the device in
the left lower lobe and a right channel extending through the
device in the right lower lobe, wherein a modulus of elasticity of
the device gradually varies across the device.
10. The device of claim 9 wherein the modulus increases from the
upper body portion to the lower body portion.
11. The device of claim 10 wherein the modulus decreases from the
left sidewall to the central body portion.
12. The device of claim 10 wherein the modulus decreases from the
right sidewall to the central body portion.
13. The device of claim 9 wherein the modulus decreases from the
left sidewall to the central body portion.
14. The device of claim 9 wherein the modulus decreases from the
right sidewall to the central body portion.
15. The device of claim 9 wherein the device includes an inner body
portion between the front face and the rear face wherein the
modulus decreases from the front face and the rear face to the
inner body portion.
Description
BACKGROUND
[0001] This invention relates generally to devices for the
treatment of spinal conditions, and more particularly, to the
treatment of various spinal conditions that cause back pain. Even
more particularly, this invention relates to devices that may be
placed between adjacent spinous processes to treat various spinal
conditions. For example, spinal conditions that may be treated with
these devices may include spinal stenosis degenerative disc disease
(DDD), disc herniations and spinal instability, among others.
[0002] The clinical syndrome of neurogenic intermittent
claudication due to lumbar spinal stenosis is a frequent source of
pain in the lower back and extremitiesi leading to impaired
walking, and causing other forms of disability in the elderly.
Although the incidence and prevalence of symptomatic lumbar spinal
stenosis have not been established, this condition is the most
frequent indication of spinal surgery in patients older than 65
years of age.
[0003] Lumbar spinal stenosis is a condition of the spine
characterized by a narrowing of the lumbar spinal canal. With
spinal stenosis, the spinal canal narrows and pinches the spinal
cord and nerves, causing pain in the back and legs. It is estimated
that approximately 5 in 10000 people develop lumbar spinal stenosis
each year. For patients who seek the aid of a physician for back
pain, approximately 12%-15% are diagnosed as having lumbar spinal
stenosis.
[0004] Common treatments for lumbar spinal stenosis include
physical therapy (including changes in posture), medication, and
occasionally surgery. Changes in posture and physical therapy may
be effective in flexing the spine to decompress and enlarge the
space available to the spinal cord and nerves--thus relieving
pressure on pinched nerves. Medications such as NSAIDS and other
anti-inflammatory medications are often used to alleviate pain,
although they are not typically effective at addressing spinal
compression, which is the cause of the pain.
[0005] Surgical treatments are more aggressive than medication or
physical therapy, and in appropriate cases surgery may be the best
way to achieve lessening of the symptoms of lumbar spinal stenosis
and other spinal conditions. The principal goal of surgery to treat
lumbar spinal stenosis is to decompress the central spinal canal
and the neural foramina, creating more space and eliminating
pressure on the spinal nerve roots. The most common surgery for
treatment of lumbar spinal stenosis is direct decompression via a
laminectomy and partial facetectomy. In this procedure, the patient
is given a general anesthesia and an incision is made in the
patient to access the spine. The lamina of one or more vertebrae
may be partially or completely removed to create more space for the
nerves. The success rate of decompressive laminectomy has been
reported to be in excess of 65%. A significant reduction of the
symptoms of lumbar spinal stenosis is also achieved in many of
these cases.
[0006] The failures associated with a decompressive laminectomy may
be related to postoperative iatrogenic spinal instability. To limit
the effect of iatrogenic instability, fixation and fusion may also
be performed in association with the decompression. In such a case,
the intervertebral disc may be removed, and the adjacent vertebrae
may be fused. A discectomy may also be performed to treat DDD and
disc herniations. In such a case, a spinal fusion would be required
to treat the resulting vertebral instability. Spinal fusion is also
traditionally accepted as the standard surgical treatment for
lumbar instability. However, spinal fusion sacrifices normal spinal
motion and may result in increased surgical complications. It is
also believed that fusion to treat various spinal conditions may
increase the biomechanical stresses imposed on the adjacent
segments. The resultant altered kinematics at the adjacent segments
may lead to accelerated degeneration of these segments.
[0007] As an alternative or complement to the surgical treatments
described above, an interspinous process device may be implanted
between adjacent spinous processes of adjacent vertebrae. The
purposes of these devices are to provide stabilization after
decompression, to restore foraminal height, and to unload the facet
joints. They also allow for the preservation of a range of motion
in the adjacent vertebral segments, thus avoiding or limiting
possible overloading and early degeneration of the adjacent
segments as induced by fusion. The vertebrae may or may not be
distracted before the device is implanted therebetween. An example
of such a device is the interspinous prosthesis described in U.S.
Pat. No. 6,626,944, the entire contents of which are expressly
incorporated herein by reference. This device, commercially known
as the DIAM.RTM. spinal stabilization system, is designed to
restabilize the vertebral segments as a result of various surgical
procedures or as a treatment of various spinal conditions. It
limits extension and may act as a shock absorber, since it provides
compressibility between the adjacent vertebrae, to decrease
intradiscal pressure and reduce abnormal segmental motion and
alignment. This device provides stability in all directions and
maintains the desired separation between the vertebral segments all
while allowing motion in the treated segment.
[0008] Although currently available interspinous process devices
typically work for their intended purposes, they could be improved.
For example, where the spacer portion of the implant is formed from
a hard material to maintain distraction between adjacent vertebrae,
point loading of the spinous process can occur due to the high
concentration of stresses at the point where the hard material of
the spacer contacts the spinous process. This may result in
excessive subsidence of the spacer into the spinous process. In
addition, if the spinous process is osteoporotic, there is a risk
that the spinous process could fracture when the spine is in
extension. In addition, because of the human anatomy and the
complex biomechanics of the spine, some currently available
interspinous process devices may not be easily implantable in
certain locations in the spine.
[0009] The spine is divided into regions that include the cervical,
thoracic, lumbar, and sacrococcygeal regions. The cervical region
includes the top seven vertebrae indentified as C1-C7. The thoracic
region includes the next twelve vertebrae identified as T1-T12. The
lumbar region includes five vertebrae L1-L5. The sacrococcygeal
region includes five fused vertebrae comprising the sacrum. These
five fused vertebrae are identified as the S1-S5 vertebrae. Four or
five rudimentary members form the coccyx.
[0010] The sacrum is shaped like an inverted triangle with the base
at the top. The sacrum acts as a wedge between the two iliac bones
of the pelvis and transmits the axial loading forces of the spine
to the pelvis and lower extremities. The sacrum is rotated
anteriorly with the superior endplate of the first sacral vertebra
angled from about 30 degrees to about 60 degrees in the horizontal
plane. The S1 vertebra includes a spinous process aligned along a
ridge called the medial sacral crest. However, the spinous process
on the S1 vertebrae may not be well defined, or may be
non-existent, and therefore may not be adequate for supporting an
interspinous process device positioned between the L5 and S1
spinous processes.
[0011] Thus, a need exists for an interspinous process device that
may be readily positioned between the L5 and S1 spinous processes.
Moreover, there is a need to provide an interspinous process device
that can provide dynamic stabilization to the instrumented motion
segment and not affect adjacent segment kinematics.
SUMMARY
[0012] A spinal implant is described herein that is particularly
adapted for placement between the spinous processes of the L5
vertebra and the S1 vertebra to provide dynamic stabilization. The
implant includes an upper saddle defined by a pair of sidewalls
joined by a bottom wall. The upper saddle sidewalls may flare
slightly outwardly away from the sagittal plane toward the top of
the implant while the bottom wall of the upper saddle may be
concavely curved. In addition, the surfaces forming the upper
saddle sidewalls and the upper saddle bottom wall extend in a
direction, from the front of the implant to the rear of the
implant, which is generally parallel to the sagittal plane. The
upper saddle is configured to receive and support the spinous
process of the L5 vertebra therein. The implant also includes a
lower saddle defined by a pair of sidewalls joined by a top wall.
The lower saddle sidewalls flare outwardly away from the sagittal
plane toward the bottom of the implant. In addition, the surfaces
forming the lower saddle sidewalls extend in a direction, from the
front of the implant to the rear of the implant, outwardly away
from the sagittal plane. The lower saddle top wall may be concavely
curved. In addition, the surface forming the lower saddle top wall
extends in a direction, from the front of the implant to the rear
of the implant, toward the top of the implant. The lower saddle is
not intended to engage and is not supported by the spinous process
of the S1 vertebra. Rather the lower saddle merely provides a space
into which that spinous process may extend when the implant is
properly located in place.
[0013] The spinal implant described herein has outer sidewalls that
extend on either side of the implant from the upper portion of the
implant to the lower portion of the implant. The outer sidewalls
flare outwardly away from the sagittal plane from the upper portion
of the implant to give the implant a generally triangular-like
shape. The wider bottom portion of the implant allows two lower
lobes to be defined along the bottom portion of the implant
adjacent to either side of the lower saddle. The lower lobes each
define a channel extending through the thickness of the implant.
The channels allow a fixation device to extend therethrough to fix
the implant in the desired location. These channels flare outwardly
so the fixation device can extend to the pedicles of the S1
vertebra. For example, the channels may extend at an angle of about
60 degrees away from the sagittal plane toward the rear of the
implant and at an angle of about 5 degrees toward the top of the
implant in a direction from the front of the implant toward the
rear of the implant.
[0014] The spinal implant described herein may also define a
passage that extends completely through the implant from one side
of the implant to the other side of the implant. The passage may
have a concavely curved trajectory when viewed from the top of the
implant such that the openings on either side of the implant are
generally aligned with a bottom portion of the upper saddle and the
nadir of the passage is below and generally aligned along the
sagittal plane with the lowest portion of the upper saddle bottom
wall and the highest portion of the lower saddle top wall. A tether
may extend through this passage. The curve of the passage
facilitates a tether being threaded through the passage.
[0015] The spinal implant described herein may be formed as a
unitary body of a polymeric material having a gradual variation in
modulus along and/or across its cross-section. For example, the
modulus may vary along the height of the body such that the upper
portion of the body is more flexible than the lower portion of the
body. Alternatively, the modulus may vary along the width of the
body such that the outer lateral portions of the body are stiffer
than the central portion of the body. The modulus may also vary
along the width and height of the body such that the outer lateral
portions and the lower portion of the body are stiffer than the
central and upper portion of the body.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] FIG. 1 is a front perspective view of one embodiment of a
lumbar-sacral implant;
[0017] FIG. 2 is a rear perspective view of the embodiment of a
lumbar-sacral implant shown in FIG. 1;
[0018] FIG. 3 is a bottom perspective view of the embodiment of a
lumbar-sacral implant shown in FIG. 1;
[0019] FIG. 4 is a rear elevation view of the embodiment of a
lumbar-sacral implant shown in FIG. 1;
[0020] FIG. 5 is a cross-sectional view of the embodiment of a
lumbar-sacral implant shown in FIG. 1 taken along line V-V in FIG.
3;
[0021] FIG. 6 is a cross-sectional view of the embodiment of a
lumbar-sacral implant shown in FIG. 5 located between the L5
spinous process and the sacrum;
[0022] FIG. 7 is a cross-sectional view of the embodiment of a
lumbar-sacral implant shown in FIG. 1 taken along line VII-VII FIG.
3;
[0023] FIG. 7A is cross-sectional view similar to the view shown in
FIG. 7 showing an alternate embodiment of the lumbar-sacral implant
shown in FIG. 1;
[0024] FIG. 8 is a side elevation view of the lumbar-sacral implant
shown in FIG.
[0025] FIG. 9 is a front elevation schematic view of the
lumbar-sacral implant shown in FIG. 1 depicting one embodiment of
the varying durometer of the implant;
[0026] FIG. 10 is a front elevation schematic view of the
lumbar-sacral implant shown in FIG. 1 depicting another embodiment
of the varying durometer of the implant;
[0027] FIG. 11 is a front elevation view of the lumbar-sacral
implant shown in FIG. 1 mounted on a spine; and
[0028] FIG. 12 is a side elevation view of the lumbar-sacral
implant shown in FIG. 1 mounted on a spine.
DETAILED DESCRIPTION
[0029] As used in this specification and the appended claims, the
singular forms "a," "an" and "the" include plural referents unless
the context clearly dictates otherwise. Thus, for example, the term
"a member" is intended to mean a single member or a combination of
members, and "a material" is intended to mean one or more
materials, or a combination thereof. Furthermore, the words
"proximal" and "distal" refer to directions closer to and away
from, respectively, an operator (e.g., surgeon, physician, nurse,
technician, etc.) who would insert the medical device into the
patient, with the tip-end (i.e., distal end) of the device inserted
inside a patient's body first. Thus, for example, the device end
first inserted inside the patient's body would be the distal end of
the device, while the device end last to enter the patient's body
would be the proximal end of the device.
[0030] As used in this specification and the appended claims, the
terms "upper", "top", "lower", "bottom", "front", "back", "rear",
"left", "right", "side", "middle" and "center" refer to portions of
or positions on the implant when the implant is oriented in its
implanted position.
[0031] As used in this specification and the appended claims, the
term "axial plane" when used in connection with particular
relationships between various parts of the implant means a plane
that divides the implant into upper and lower parts. As shown in
the FIGS., the axial plane is defined by the X axis and the Z axis.
As used in this specification and the appended claims, the term
"coronal plane" when used in connection with particular
relationships between various parts of the implant means a plane
that divides the implant into front and back parts. As shown in the
FIGS., the coronal plane is defined by the X axis and the Y axis.
As used in this specification and the appended claims, the term
"sagittal plane" when used in connection with particular
relationships between various parts of the implant means a plane
that divides the implant into left and right parts. As show in the
FIGS., the sagittal plane is defined by the Y axis and the Z
axis.
[0032] As used in this specification and the appended claims, the
term "body" when used in connection with the location where the
device of this invention is to be placed to treat spinal disorders,
or to teach or practice implantation methods for the device, means
a mammalian body. For example, a body can be a patient's body, or a
cadaver, or a portion of a patient's body or a portion of a
cadaver.
[0033] As used in this specification and the appended claims, the
term "parallel" describes a relationship, given normal
manufacturing or measurement or similar tolerances, between two
geometric constructions (e.g., two lines, two planes, a line and a
plane, two curved surfaces, a line and a curved surface or the
like) in which the two geometric constructions are substantially
non-intersecting as they extend substantially to infinity. For
example, as used herein, a line is said to be parallel to a curved
surface when the line and the curved surface do not intersect as
they extend to infinity. Similarly, when a planar surface (i.e. t a
two-dimensional surface) is said to be parallel to a line, every
point along the line is spaced apart from the nearest portion of
the surface by a substantially equal distance. Two geometric
constructions are described herein as being "parallel" or
"substantially parallel" to each other when they are nominally
parallel to each other, such as for example, when they are parallel
to each other within a tolerance. Such tolerances can include, for
example, manufacturing tolerances, measurement tolerances or the
like.
[0034] As used in this specification and the appended claims, the
terms "normal", "perpendicular" and "orthogonal" describe a
relationship between two geometric constructions (e.g., two lines,
two planes, a line and a plane, two curved surfaces, a line and a
curved surface or the like) in which the two geometric
constructions intersect at an angle of approximately 90 degrees
within at least one plane. For example, as used herein, a line is
said to be normal, perpendicular or orthogonal to a curved surface
when the line and the curved surface intersect at an angle of
approximately 90 degrees within a plane. Two geometric
constructions are described herein as being "normal",
"perpendicular", "orthogonal" or "substantially normal",
"substantially perpendicular", "substantially orthogonal" to each
other when they are nominally 90 degrees to each other, such as for
example, when they are 90 degrees to each other within a tolerance.
Such tolerances can include, for example, manufacturing tolerances,
measurement tolerances or the like.
[0035] A spinal implant 10 is described herein that is particularly
adapted for placement between the spinous processes of the L5
vertebra and the S1 vertebra. However, it is to be understood that
even though the following description of implant 10 is provided
with reference to the L5 spinous process and the S1 spinous
process, implant 10 may be used between other adjacent spinous
process and the discussion of the L5 spinous process may be
interpreted to include any superior spinous process and the S1
spinous process may be interpreted to include the adjacent inferior
spinous process.
[0036] Implant 10 includes an upper saddle 20 defined by a pair of
sidewalls 21a and 21b joined by a bottom wall 22. Upper saddle 20
receives and supports the spinous process of the L5 vertebra
therein. Upper saddle sidewalls 21a and 21b may flare slightly
outwardly away from the sagittal plane toward the top of implant 10
while upper saddle bottom wall 22 may be concavely curved. Implant
10 may have a variable radius (from about 3.0 mm on the ventral
face 12 to about 2.0 mm on the dorsal face 45. This allows implant
10 to engage the L5 spinous process, which is usually thicker at
the base. As shown in FIG. 5, upper saddle 20 may be oriented at
about a 10 degree angle in the sagittal plane. The angle could be
as large as about degrees. The surfaces forming upper saddle
sidewalls 21a and 21b and upper saddle bottom wall 22 may be
generally parallel to the sagittal plane. This configuration for
upper saddle 20 allows upper saddle 20 to receive and support the
spinous process of an L5 vertebra therein. The height of upper
saddle sidewalls 21a and 21b should be chosen so that upper saddle
sidewalls 21a and 21b prevent the upper portion of implant 10 from
moving laterally out of engagement with the spinous process of the
L5 vertebra. Upper saddle sidewalls 21a and 21b may extend between
1/3 and 1/2 of the base of the spinous process so they engage the
lamina by about 2 to 3 mm. Upper saddle sidewalls 21a and 21b may
not have a constant cross-section. This would allow upper saddle 20
to accommodate the variable thickness of the spinous process.
Implant 10 also includes a lower saddle 30 defined by a pair of
sidewalls 31a and 31b joined by a top wall 32. As described in more
detail below, lower saddle 30 has a configuration to provide
clearance of implant 10 over the S1 spinous process. As such, lower
saddle 30 would not engage the spinous process of the S1 vertebra.
Lower saddle sidewalls 31a, and 31b flare outwardly away from the
sagittal plane toward the bottom of implant 10.
[0037] Upper saddle sidewalls 21a and 21b flare out and have a
variable angle. The angle starts at about 40 degrees at the upper
portion of upper saddle 20 and varies so that the angle is about 25
degrees at about the lowermost portion of upper saddle 20. Lower
saddle sidewalls 31a and 31b flare out and have a constant angle
between about 25 degrees and about 35 degrees. Lower saddle top
wall 32 may be concavely curved or may have another configuration
that allows the lower portion of implant 10 to be fixed to the S1
pedicles and minimizes any interference between the S1 spinous
process and the rear of implant 10. Lower saddle top wall 32 is
inclined between about 30 degrees and about 35 degrees in the
sagittal plane.
[0038] Implant 10 has outer sidewalls 11a and 11b that extend on
either side of implant 10 from the upper portion of implant 10 to
the lower portion of implant 10. Outer sidewalls 11a and 11b flare
outwardly away from the sagittal plane from the upper portion of
implant 10 to give implant 10 a generally triangular shape. In
addition, the overall shape of implant 10 transfers load from the
L5 spinous process to the S1 pedicles instead of to the S1 spinous
process or the S1 laminae. This is especially helpful where implant
10 is used in the L5-S1 level since the small size and shape of the
S1 spinous process may not provide adequate support for an
implant.
[0039] The front face 12 of implant 10 may have a curved profile
that tapers from about 0 degrees along the middle of front face 12
to about 35 degrees adjacent to sidewalls 11a, 11b. Implant 10 may
have a curvature radius of between about 20 mm and about 30 mm. The
generally triangular shape, where the base is larger than the top
results in a constant pressure applied along the cross-sectional
area of implant 10. The shape of implant 10 also provides a better
fit in the L5/S1 space and therefore offers stability for implant
10. The rear of implant 10 has a stepped configuration and includes
a shelf 40 separating the rear of implant 10 into an upper portion
and a lower portion. Shelf 40 may be curved and is located so it is
generally aligned with or above channels 34a and 34b. Shelf 40 acts
as a transition between the upper and lower portions of the rear of
implant 10 and ensures that implant 10 will fit properly in the
patient's anatomy. The upper rear portion of implant 10 is defined
by the rear wall 45, which flares outwardly from the top of implant
10. Rear wall 45 is curved such that it does not compete for
engagement with upper saddle 20 but rather allows implant 10 to
rest freely on the L5 lamina. This allows for easy implantation on
the L5 level. The thickness of implant 10 gradually increases from
the top of implant 10 to shelf 40. This taper may be between about
30 degrees and about 50 degrees. The bottom rear portion of implant
10 has a thinner profile and provides clearance so that lower
saddle 30 does not engage the inferior spinous process. This
results in practically no load being transferred from implant 10 to
the inferior spinous process. Indeed, lower saddle 30 may be
configured such that it is spaced from and does not engage the
inferior spinous process when implant 10 is implanted in the
patient.
[0040] The wider bottom portion of implant 10 allows two lower
lobes 33a and 33b to be defined along the bottom portion of implant
10 adjacent to either side of lower saddle 30 and provides an area
through which implant 10 may be fixed to the spine. The wider
bottom portion of implant 10, and indeed the overall configuration
of implant 10, also allow implant 10 to withstand higher forces
being placed on it and helps to ensure compression forces placed on
implant 10 are evenly distributed throughout the body of implant
10.
[0041] Each lower lobe 33a and 33b defines a channel 34a and 34b
extending through implant 10. Channels 34a and 34b allow a fixation
device 60, such as a cortical screw or similar device, to extend
therethrough to fix implant 10 in the desired location on the
spine. As such, the internal diameter of channels 34a and 34b
should be sufficient to allow passage of fixation device 60
therethrough, but should not be so large as to allow too much
"play", or too big of a gap, between fixation device 60 and
channels 34a and 34b. For example, channels 34a and 34b could have
an internal diameter that is about 0.5 mm to about 1 mm greater
than the outer diameter of fixation device 60. Channels 34a and 34b
flare outwardly from about the mid-line of implant 10 and adjacent
to the top of the bottom portion of implant 10 so that fixation
device 60 can be located therein and extend to the pedicles of the
S1 vertebra. For example, channels 34a and 34b may extend at an
angle .alpha. of about 60 degrees away from the sagittal plane
toward the rear of implant and at an angle .beta. of about 5
degrees toward the top of implant 10 in a direction from the front
of implant 10 toward the rear of implant 10. Alternatively, angle
.alpha. could be between about 45 degrees and about 60 degrees,
while angle .beta. could be between about 5 degrees and about 10
degrees. The wider bottom portion of implant 10, and indeed the
overall configuration of implant 10, also allow implant 10 to
withstand higher forces being placed on it and helps to ensure
compression forces placed on implant 10 are evenly distributed
throughout the body of implant 10.
[0042] Implant 10 may be formed as a unitary body of a polymeric
material having a gradual variation in the modulus of elasticity
along and/or across its cross section. For example, the modulus may
vary along the height of the body such that the upper portion of
the body is more flexible than the lower portion of the body. See
for example FIG. 9, which shows this arrangement schematically.
Alternatively, the modulus may vary along the width of the body
such that the outer lateral portions of the body are stiffer than
the central portion of the body. The modulus may also vary along
the width and height of the body such that the outer lateral
portions and the lower portion of the body are stiffer than the
central and upper portion of the body. See for example FIG. 10,
which shows this arrangement schematically.
[0043] A material that may be formed into a product having a
gradual variation in the modulus of elasticity, and the process for
making the material, are disclosed in pending published U.S. Patent
Application No. 2007/0050038, the entire contents of which are
hereby expressly incorporated herein by reference. The material and
process disclosed in the '038 Publication would allow the inferior
portion of implant 10, or lower lobes 33a and 33b, to be relatively
stiff, i.e. have a higher modulus of elasticity, so that fixation
device 60 can firmly affix implant 10 to the spine while ensuring
that the inferior portion, or lower lobes 33a and 33b, will not be
pulled from fixation device 60 during flexion or other movement of
the spine. Such pulling through of the implant is more likely if
the inferior portion, or lower lobes 33a and 33b, were formed from
a flexible material. Conversely, the material and process disclosed
in the '038 Publication would allow the superior portion to be more
elastic and flexible, i.e. have a lower modulus of elasticity.
Having the superior portion be more flexible allows implant 10 to
act as a shock absorber in extension while providing adequate
stabilization to the L5/S1 level and allowing a more normal range
of motion. In addition to having a gradual increasing of the
modulus of elasticity from the top of implant 10 to the bottom of
implant 10, the modulus of elasticity could vary from the surface
of implant 10 to the inner core of implant 10 such that the inner
core would have a lower modulus of elasticity than the surface. In
this way, the inner core would be more flexible than the surface.
In addition, the modulus of elasticity could vary from the top and
central portions of implant 10 to the sides and bottom of implant
10 such that the sides and bottom portion of implant 10 would have
a higher modulus of elasticity than the top and central portions.
In this way, the sides and bottom would be stiffer than the top and
central portions.
[0044] Implant 10 may also define a curved passage 80 that extends
between outer sidewalls 11a and 11b of implant 10. The curve of
passage 80 may be defined by a radius of curvature of about 20
millimeters where the openings 85a and 85b to passage 80 are closer
to the top of implant 10 than the nadir of passage 80. Openings 85a
and 85b are generally perpendicular to outer sidewalls 11a and 11b.
Other radii of curvature may also be used to define passage 80. The
nadir of passage 80 may be substantially aligned in the sagittal
plane with the bottom most portion of upper saddle bottom wall 22
and the uppermost portion of lower saddle top wall 32. A tether 90
may extend through passage 80. The curve of passage 80 facilitates
tether 90 being threaded through passage 80 with a standard curved
surgical needle. As shown in FIGS. 11 and 12, tether 90 may extend
across the superior portion of the superior spinous process when
implant 10 is located in the interspinous space. Tether 90 thus
helps to maintain implant 10 in the proper position in the
patient's anatomy during extension and flexion. It is to be
understood that other fixation devices may be used instead of a
tether 90. For example, a pin, rod, screw or other similar
mechanical device may be used and would extend through upper saddle
20 and into the upper spinous process.
[0045] While various embodiments of the flexible interspinous
process device have been described above, it should be understood
that they have been presented by way of example only, and not
limitation. Many modifications and variations will be apparent to
the practitioner skilled in the art. The foregoing description of
the flexible interspinous process device is not intended to be
exhaustive or to limit the scope of the invention. It is intended
that the scope of the invention be defined by the following claims
and their equivalents.
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